This invention relates generally to optical devices for altering the distribution of radiant energy and, more particularly, to optical devices for altering the spatial and/or angular distribution of electromagnetic radiation between an input plane and an output plane.
An energy concentrator is an optical device for increasing, between an input plane and an output plane, the energy density of radiant energy. One instance of a concentrator is a mirror or lens that focuses incident radiation to a relatively small area in the concentrator's focal or target plane. The resulting irradiance distribution often has a peaked shape. An energy concentrator generally is designed to maximize the amount of incident radiation that is directed onto a target area. By way of example, the irradiance distribution in the focal plane of a 10 kilowatt solar concentrator is shown in FIGS. 15A and 15B. The peaked distribution limits the possible applications of the concentrated irradiance.
In some applications, a uniform irradiance distribution over a specified target region is desired while simultaneously maintaining as high a mean irradiance level as possible. It has been found that defocusing the concentrator, by placing the target area of interest in front of or behind the focal plane, fails to provide a uniform irradiance distribution, as shown in FIGS. 16A and 16B. It will be observed that, although the irradiance distribution in the focal plane exhibits a well formed peaked distribution, the unfocused beam exhibits wide intensity differences across the target area. If the target area is an absorber of the concentrated energy, it must be able to withstand extreme thermal stresses in the various highly localized regions associated with the intensity peaks in the defocused irradiance distribution. At high irradiance levels, some absorbers of interest cannot withstand these high thermal stresses. Also, it is typically inefficient to configure an absorber to accommodate these stresses.
Theoretically, a long tube having a highly reflective inner surface, when a nonuniform irradiance distribution is input at one end, can output a uniform irradiance distribution at the other end. However, the use of such a tube is largely impractical because of the long length required and because of large reflective losses due to the multiple reflections occurring in the tube.
Accordingly, there is a need for a relatively compact device having a reflective surface that distributes, in a predetermined manner, the irradiance from a radiation source over a predetermined target area. The present invention satisfies this need.